Natural cytotoxicity of murine cytomegalovirus-infected cells mediated by mouse lymphoid cells: role of interferon in the endogenous natural cytotoxicity reaction

Animal cytomegaloviruses

Cytomegaloviruses are agents that infect a variety of animals. Human cytomegalovirus is associated with infections that may be inapparent or may result in severe body malformation. More recently, human cytomegalovirus infections have been recognized as causing severe complications in immunosuppressed individuals. In other animals, cytomegaloviruses are often associated with infections having relatively mild sequelae. Many of these sequelae parallel symptoms associated with human cytomegalovirus infections. Recent advances in biotechnology have permitted the study of many of the animal cytomegaloviruses in vitro. Consequently, animal cytomegaloviruses can be used as model systems for studying the pathogenesis, immunobiology, and molecular biology of cytomegalovirus-host and cytomegalovirus-cell interactions.

The potential usefulness of interleukin-2 activated bone marrow cells as an active therapeutic tool against cytomegalovirus infection in a bone marrow transplantation setting

Bone marrow transplantation (BMT) has been used in recent years for the treatment of immunodeficiency diseases, aplastic anemia, and leukemia. However, there are a number of serious problems and limitations associated with autologous or allogeneic BMT. One of these is an increase in opportunistic infections, of which cytomegalovirus (CMV) infection is one of the most important. Cytomegalovirus has been associated with more frequent deaths than any other single agent, with no reproducibly successful or therapy currently available. Recently usage of interleukin-2 or immunomodulation has been suggested as a powerful modality to combat infectious disease. In this study we showed that bone marrow activated in interleukin-2 for 2 days has the ability to lyse spleen cells infected for 3 days with murine CMV (acute infection model) or salivary gland cells infected for 7 days (chronic infection model), while nonactivated bone marrow or natural killer (NK) cells showed no such lysis. The majority of activated cells involved in lysis were antiasialo GM1-, Thy-1+/-, indicating a population of cells other than the natural killer-cell population involved.

Molecular biology and immunology of cytomegalovirus

The application of modern biochemical techniques has led to a rapid improvement in our knowledge of the molecular biology of CMV. Several coding regions of the DNA genome have been identified with certainty and major virus-coded proteins have been given provisional names. The cascade expression of the CMV genome has been shown to be controlled by mechanisms similar to those found in other herpes viruses, together with novel post-transcriptional controls which remain to be defined. The control of CMV replication by the host involves both non-specific and specific defence mechanisms. The induction of natural killer cells and interferon early after CMV infection appears to be the most important aspects of the non-specific host defence against the virus. The cell-mediated immune response, in particular the generation of Tc cells against CMV early antigens, is probably the most important facet of the specific immune defence against CMV. When intact these defence mechanisms appear to be efficient in restricting viral replication; however, when such immunity is compromised, the balance rapidly swings in favour of the virus. As our understanding of the interaction between the host and the virus increases, it may be possible to redress the balance in such cases in favour of the host.

The role of NK cell activity in age-dependent resistance of mice to murine cytomegalovirus infection

The resistance of mice to cell culture passaged murine cytomegalovirus (CC-MCMV) infection developed with age. In parallel with this finding, augmentation of the splenic NK cell activity in older mice was always higher than that of younger mice. The splenic NK cell activity reached the maximum level at 6 day post infection (PI) in 2-4-week-old mice while in 6-8-week-old mice it peaked at 4 days PI. When the dose of CC-MCMV was increased, the NK cell activity was potentiated accordingly. However, it was decreased on the infection with increased doses of the salivary gland passaged MCMV (SG-MCMV). NK cells augmented by MCMV infection actually inhibited in vitro replication of MCMV when they were added to mouse embryonic fibroblast (MEF) monolayers infected with CC-MCMV. Splenic and peritoneal macrophages inhibited in vitro replication of MCMV, but their activities were less potent than those of NK cells.

Cytomegalovirus (CMV)-specific lysis of CMV-infected target cells can be mediated by both NK-like and virus-specific cytotoxic T lymphocytes

The host response to cytomegalovirus (CMV) involves both humoral and cell-mediated immunity. Because virus-specific cytotoxicity appears to be associated with recovery from CMV infection, we have investigated spontaneous (NK) and cytotoxic T cell (Tc) activity against CMV-infected target cells in normal donors and infants with active CMV infection. Fresh mononuclear cells from seropositive donors expressed low-level, non-MHC-restricted cytotoxic activity that preferentially lysed CMV-infected but not uninfected autologous and allogeneic fibroblasts. No cytotoxic activity was observed when mononuclear cells from seronegative donors were studied. Mononuclear cells from infants with active CMV infection and mononuclear cells from seropositive adults that were pre-incubated with cell-free CMV antigen in bulk culture for 6 days had enhanced cytotoxic activity against CMV-infected target cells sharing one or more class I MHC determinants with the effector cell populations. Selective enrichment or depletion experiments were performed to characterize the effector cell populations involved in the augmented cytotoxic response following in vitro induction with CMV antigen. Purified E-rosette-forming cells expressed increased cytotoxic activity against both virus-infected fibroblasts and the NK target cell, K562. Depletion experiments with monoclonal antibodies and complement indicated that the CMV-specific cytotoxic response involves both an E-rosette-positive, T8+, M1- T-cell subset that most likely represents CMV-specific Tc, and an NK population that can be induced to differentiate by in vitro stimulation with CMV antigen following quantitative NK-cell depletion with monoclonal antibody, B73.1.

In vitro and systemic effects of recombinant bovine interferons on natural cell-mediated cytotoxicity in healthy and bovine herpesvirus-1-infected cattle

There is now evidence to suggest that the beneficial effect of treatment of calves with recombinant bovine interferons (IFNs) on the outcome of an infection with bovine herpes-virus-1 (BHV-1) may in part be due to effects other than its direct antiviral activity. This evidence prompted us to study the effect of IFNs on natural cell-mediated cytotoxicity (NC) as a possible mechanism in curtailing clinical disease. In vitro treatment of blood leukocytes not only augmented cytotoxicity to NC-susceptible target cells, but also to cell types usually resistant to killing. In vivo treatment of healthy cattle with IFN-gamma caused a transient rise in NC activity, which was both dose and route dependent. Treatment of calves with IFN-alpha 1 or IFN-gamma prior to BHV-1 infection prevented or diminished the suppression of NC activity usually seen following virus infection. The effect could neither be correlated with serum IFN titers nor with conventional hematologic parameters. A possible mechanism for the IFN effect involving the ontogeny of bovine NC-effector cells and the biological consequences for the local antiviral defense in the lung are discussed.

Inhibition of antibody-dependent cell-mediated cytotoxicity by ethanol

Ethanol has been shown to inhibit spontaneous cell-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity in vitro in a dose-dependent manner. The effect was observed under differing assay conditions such as incubation time and effector: target cell ratio. Acetaldehyde, the primary metabolite of ethanol, did not inhibit these functions of the immune system. A mixture of ethanol and acetaldehyde showed no interaction with respect to spontaneous and antibody-dependent cell-mediated cytotoxicity. These observations may help to explain the apparently increased incidence of infection for individuals who chronically ingest large amounts of alcoholic beverages.

Pathogenesis of murine cytomegalovirus infection in natural killer cell-depleted mice

The effect of natural killer (NK) cells on the course of acute and persistent murine cytomegalovirus (MCMV) infection was examined by selectively depleting NK cell activity by inoculation of mice with antibody to asialo GM1, a neutral glycosphingolipid present at high concentrations on NK cells. The dose of MCMV required to cause 50% mortality or morbidity in control C57BL/6 mice dropped 4- and greater than 11-fold, respectively, in mice first treated with anti-asialo GM1. NK cell-depleted mice had higher (up to 1,000-fold) virus titers in their lungs, spleens, and livers at days 3, 5, 7, and 9 postinfection. Spleens and livers of control mice were virus-free by day 7 postinfection, and their lungs showed no signs of active infection at any time. In contrast, MCMV had disseminated to the lungs of NK cell-depleted mice by day 5, and these mice still had moderate levels of virus in their lungs, spleens, and livers at day 9. Markedly severe pathological changes were noted in the livers and spleens of NK cell-depleted, MCMV-infected mice. These included ballooning degeneration of hepatocytes and spleen necrosis. MCMV-infected, NK cell-depleted mice had severe spleen leukopenia, and their spleen leukocytes exhibited a significantly lower (up to 13-fold) response to the T cell mitogen concanavalin A when compared with those of uninfected and MCMV-infected controls. It appeared that NK cells exerted their most potent antiviral effect early in the infection, in a pattern correlating with interferon production and NK cell activation; treatment with anti-asialo GM1 later in infection had no effect on virus titers. The relative effect of NK cell depletion on MCMV pathogenesis depended on the injection route of the virus. NK cell depletion greatly augmented MCMV synthesis and pathogenesis in mice inoculated either intravenously or intraperitoneally but had no effect on the course of disease after intranasal inoculation, at any time point examined. One month after intraperitoneal inoculation of virus, NK cell depletion resulted in a six- to eightfold increase in salivary gland virus titers in persistently infected mice, suggesting that NK cells may be important in controlling virus synthesis in the salivary gland during persistent infection. This treatment did not, however, induce dissemination of virus to other organs. These data support the hypothesis that NK cells limit the severity, extent, and duration of acute MCMV infection and that they may also be involved in regulating the persistent infection.

Protective effect of biological response modifiers on murine cytomegalovirus infection

Pretreatment with two biological response modifiers (BRM), OK-432 and PS-K, protected mice from lethal infection by murine cytomegalovirus (MCMV). This was evidenced by an increase in 50% lethal doses and a decrease in titers of infectious viruses replicated in the liver and spleen. Spleen cells from the BRM-treated mice augmented the natural killer (NK) cell activity and suppressed the replication of MCMV in vitro. During MCMV infection, the NK cell activity of the spleen cells was maintained at a high level in the BRM-treated mice, whereas it was severely impaired in untreated mice. The BRM-induced protection was nullified by concomitant administration of antiasialo GM1 antibody. Interferon was neither induced by BRM treatment nor enhanced in BRM-pretreated and MCMV-infected mice. Thus, the protective effect of OK-432 and PS-K seems to be based on activation of NK cells and prevention of MCMV-induced inhibition of the NK cell activity.

Influence of one virus infection on a second concurrent primary in vivo antiviral cytotoxic T-cell response

The influence of one virus on the in vivo cytotoxic T-cell response to a different concurrent viral infection was analyzed. Lymphocytic choriomeningitis and Newcastle disease viruses, known to induce high interferon titers, and the synthetic interferon inducer polyriboinosinic acid-polyribocytidylic acid inhibited the cytotoxic T-cell response against the second virus. In contrast, vaccinia and vesicular stomatitis viruses failed to induce inhibition. Inhibition directly correlated with the interferon titers; similarly, the interferon titers directly correlated with macrophage and natural killer cell activation. The involvement in vivo of interferon in macrophage and natural killer cell activation and the possible mechanisms of inhibition of the cytotoxic responses are shown by the inhibition of the effect by antibodies against interferon.

Relationship between expression of herpes simplex virus glycoproteins and susceptibility of target cells to human natural killer activity

Cells normally insensitive to human natural killer (NK) activity were rendered susceptible by infection with HSV-1. The cytotoxic effector cell was a nonadherent, non-T, non-B lymphocyte. Antibody plus complement treatment, using a monoclonal antibody that recognizes an antigen present on NK cells, removed much of the cytotoxic activity, and a density gradient fraction enriched for NK cells yielded cells of increased virus-specific cytotoxicity. It was concluded that the effector cell active against infected targets possessed characteristics of an NK cell. Blockage of viral protein synthesis during infection inhibited development of increased susceptibility of infected targets to NK activity. When targets were infected with a mutant virus unable to produce viral glycoprotein C (gC), NK activity against these targets was reduced approximately 30% compared with activity against targets infected with wild-type virus. Similarly, activity against targets infected in the presence of 2-deoxyglucose (2dG), which prevents cell surface expression of viral glycoprotein B (gB), was also reduced approximately 30%. An approximately 60% reduction in activity was seen against targets infected with mutant virus in the presence of 2dG; these targets express gD, but neither gB nor gC. When cells expressing various combinations of HSV-1 glycoproteins were used as both labeled targets and cold target competitors, it was found that the susceptibility of a particular target to NK activity was paralleled by its ability to act as a cold target competitor. This indicates that targets with decreased sensitivity to NK cells were less able to bind NK effectors. Further, the amount of interferon produced in co-cultures of NK effectors and infected target cells did not directly correlate with the amount of NK activity generated, and interferon pretreatment of effectors did not decrease virus-specific cytotoxicity. The present results suggest that HSV-1 glycoproteins expressed at the surface of infected targets may act as recognition structures for NK cells.